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Winter
The most complex season without a doubt to forecast in the British Isles is Winter as there are so many factors that can influence the weather elements of that season. You would first think that because Winter has the most methodology to consider, it's probably the easiest season to achieve accurate forecasting on in comparison to others. However, it's not easy to find a typical setup for the given methodology as some methods can cancel out the others and sometimes, we can have a very unique Winter season with some never done before actions of the atmosphere, like Winter 2016/17. Other times, you can be very lucky on forecasting the Winter season ahead but there's a low chance that you'll be right as one difference in the methodology could make all the changes to everything. It is for all these reasons why Winter is the most complex season to forecast in the British Isles. Methodology in forecasting the Winter season North Atlantic Oscillation The North Atlantic Oscillation (NAO) is an index that tells you the state of the atmosphere over the North Atlantic by the strength of the Azores High and the Icelandic Low. When this index is positive, it tells you that the Azores High and the Icelandic Low are strong. The effects of these depend on how strong they are, the state of other methodology as well as season. When the index is negative, it tells you that the Azores High and the Icelandic Low are weak or non-existent with northern blocking taking place over the Arctic Circle. In Winter, the NAO is vital to predicting our weather here in the British Isles. When it's positive, the weather is normally mild and wet or mild and dry. This is because the Azores High is strong and builds to the south of Europe ridging into southern parts of England. Meanwhile, the Icelandic Low is strong too centered over Iceland and pushing into the northwest of Scotland and Ireland. This leaves us in a southwesterly airstream which is very mild in Winter. This is our typical pattern. When the NAO's negative, the weather is normally cold and dry, cold and wet or cold and snowy. This is because northern blocking is over the Arctic Circle allowing cold air to descend into the mid-latitudes. The Icelandic Low has pushed to the south of Europe courtesy of the Polar Jet sinking further southwards than usual meaning the Azores High is non-existent or very weak. The NAO does not drive the weather to us, it's just a way of indicating the setup and seeing how likely something is to happen for us like how likely northern blocking is to take place or how likely the Azores High is going to be very strong etc. You do not need a positive NAO to be mild however as a negative NAO can have a mild or very mild Winter, just to a lesser degree than for the Winter to be cold with a negative NAO. For example, Winter 1997/98 had a negative NAO and was a very mild Winter with a CET of 6.1c (which is +2.3c above the 1981-2010 average), one of the warmest Winters in the past 40 years. It's the same with cold Winters that you don't need a negative NAO to get it cold. For example, Winter 1990/91 had a positive NAO and was a cold Winter with a CET of 3.0c (which is -0.8c below the 1981-2010 average) and had a notably cold February. This was courtesy of a strong Scandinavian High with the winds coming in from the east. The positive NAO led to the snowy conditions that happened in February 1991. The North Atlantic Oscillation is not a driver, it is only there to indicate what the atmosphere is like over the North Atlantic Ocean. There does not have to be a negative NAO for cold weather to take place in Europe, just as much as there does not have to be a positive NAO for warm weather to take place - there are multiple factors which influence so and the state of the North Atlantic Ocean is one of them. The North Atlantic Oscillation fluctuates a lot from season to season, month to month etc and does not follow any pattern though there can be some amusing coincidences you may come across when looking at its data. Arctic Oscillation The Arctic Oscillation (AO) is an index that tells you the state of the atmosphere over the Arctic Circle. When this index is positive, it tells you there is low pressure over the Arctic Circle and the Polar Vortex, which is the ribbon of air high up in the atmosphere that traps the cold in the Arctic from descending into the mid-latitudes, is intensified. This normally means the westerlies are stronger and in Winter, wet and windy conditions are the most likely scenario. When this index is negative, it tells you there is high pressure over the Arctic Circle. Depending on the strength of the high pressure, the Polar Vortex is either displaced, obliterated or weakened during this phase of the Arctic Oscillation. As the Polar Vortex is displaced, obliterated or weakened, the cold is free to descend into the mid-latitudes. However, where the cold ends up is heavily dependent on where the blocking sits in the Arctic Circle. Most of the time, the blocking is on the side of North America. As the cold descends into the USA, it meets the warm air from the south and they don't contrast well which leads to an intensified Polar Jet and results in stronger storms or depressions for us here in the British Isles. Other times, the blocking is over Greenland and Iceland on the side of the British Isles which brings cold conditions for us. On rare occasions, the blocking can be very stubborn and widespread over the Arctic Circle affecting multiple parts of the Northern Hemisphere. Rare occasions include the famous cold Winters 1962-63 & 2009-10. The Arctic Oscillation is not a driver, just like the North Atlantic Oscillation, it is only there to indicate what the atmosphere is like over the Arctic Circle. There does not have to be a negative AO for cold weather to take place in the Northern Hemisphere, just as much as there does not have to be a positive AO for warm weather to take place - there are multiple factors which influence so and the state of the Arctic Circle is one of them. The Arctic Oscillation fluctuates a lot from season to season, month to month etc and does not follow any pattern though there can be some amusing coincidences you may come across when looking at its data.